This invention relates to a semiconductor laser, particularly, to a semiconductor laser wherein an active layer, and first and second semconductor layers having said active layer therebetween are respectively formed from a conductive semiconductor layer of the same kind, and a P-N junction portion is formed in the active layer by an inverted diffusion layer.
As is known, the semiconductor laser comprises a laser medium formed from a semiconductor crystal having a double hetero junction construction. According to the basic structure of the semiconductor crystal, a first semiconductor layer, an active layer and a second semiconductor layer are laminated on a substrate in said order to form a double hetero junction construction. That is, the forbidden gaps of the first and second semiconductor layers are greater than that of the active layer. In this junction construction, carriers injected from the first and second semiconductor layers into the active layer are confined in the active layer. As the result, recombination of the carriers in the active layer is carried out effectively. That is, laser oscillation takes place in the active layer.
To enhance the efficiency of recombination of the carrier in the active layer, it is necessary to increase density of the carrier injected into the active layer. The semiconductor layer is of a stripe construction. This stripe construction has its function to guide the carrier injected into the second semiconductor so that the carrier may be concentrated in a specific region of the active layer. Thereby, the active layer is formed with a web-like radiation region the longitudinal direction of which is controlled by the crystal ends and the lateral direction controlled by stripe width.
Since the refractive index of the active layer is greater than that of the first and second semiconductor layers, the light beam generated by recombination of the carrier is confined in the radiation region. Since both longitudinal ends (i.e. crystal ends) of the radiation region form a Fabry-Perot resonance surface, the light beam is subjected to resonance amplification in the radiation region, a part of which is put out. This is the known laser beam.
The laser oscillation in the radiation region has a longitudinal mode generated in a longitudinal direction and a lateral mode generated in a lateral direction. Preferably, these are single modes. It is possible for the longitudinal mode to take a single mode due to the FabryPerot resonance surface. However, the laternal mode depends the stripe construction. That is, the stripe construction must be the construction which can prevent propagation of the light beam in the lateral direction.
Incidentally, various kinds of stripe constructions which are intended to form the lateral mode into a single mode have been proposed. However, all of these are complicated to manufacture. For example, there is a construction in which a portion is irradiated to a region from the second semiconductor layer to the first semiconductor layer to form a high resistant layer. That is, this is the construction in which the high resistant layers are formed on both sides which leave a narrow stripe width. In this case, the manufacturing step of the semiconductor crystal involves a unique step which is the irradiation of proton, lacking in consistency. Further, the known transverse-junction stripe-geometry laser can be improved in consistency of manufacture, but it has a plurality of masking steps which form a diffusion layer and is complicated.
The foregoing considerations are based on demands for reduction in threshold current value and unification of oscillation mode in addition to the demand for higher output. That is, there is the problem of breakdown of crystal ends (Fabry-Perot resonance surfaces), and there is naturally a limitation in the increase the oscillation output by merely increasing a driving current.
In view of the foregoing, a semiconductor laser has been proposed in which the above-described stripe construction is employed to form a plurality of radiation regions in an active layer, and output light beams are totalized to provide a higher output.
However, in such a semiconductor laser as described above, since the stripe construction is added, a plurality of masking steps is required, as a consequence of which the manufacturing process becomes cumbersome. In addition, in the semiconductor laser of the type described, it is desirable that the spacing between the radiation regions be narrow. However, the spacing between the radiation regions can not be made smaller than the spacing limited by the photolithographic technique. Accordingly, it is difficult to provide a higher density radiation region.
Finally, there is a demand for development of a semiconductor laser which is provided with a function of scanning output light beams, as a function of the element itself.
As is known, in photo-electronic devices such as facsimilies, a photodeflector composed of electric optical elements, a movable mirror and the like are combined to scan the output light beams of the semiconductor laser.
However, in making the combination with the photodeflector, there are many problems which are hard to solve due to the complicated mechanism. To make effective use of advantages of the semiconductor laser, which are smallness in size and lightness in weight, which constitute the significant characteristics thereof, development of a semiconductor laser provided in the element itself with the function scanning output light beams has been eagerly waited.